How safe are peat‐free growing media? An exploration of plant pathogen risks to the horticultural industry and recommendations for risk mitigation

Abstract

Societal Impact Statement The UK's horticultural industry is shifting from reliance on unsustainable peat‐based growing media to products that contain alternative constituents, including coir, bark, wood fibre and composted green waste. Some peat‐alternatives carry an elevated risk of harbouring plant pathogens, including the potential to introduce novel pathogens from overseas, threatening the biosecurity of UK horticulture which relies on healthy plants. Our findings support regulation for clarity on constituent sources, defined national sanitisation standards and routine testing for pathogens and provide evidence to underpin the future development of a quality assurance scheme for UK horticultural growing media currently being postulated by the industry. Summary The use of peat in horticultural growing media is being phased out in the UK due to environmental concerns. Recent evidence suggests elevated plant pathogen risks associated with peat alternatives; an uncertainty compounded by a lack of information on the source, production and processing methods for the constituents of peat‐free growing media. This study was conducted to fill these knowledge gaps. Through literature review and interviews with industry stakeholders, we present information on the size and nature of the UK professional and amateur growing media sectors, define the primary constituents of peat‐free growing media, their origin, processing methods and associated pathogen risks. Coir, softwood pine bark, wood fibre and composted organic wastes are the main alternative constituents to peat. For all constituents, there is a lack of standardisation of sanitisation regimes and an absence of critical limits for key sanitisation parameters. Constituents of the highest biosecurity risk are coir imported by smaller companies from small producers in Asian countries, use of recycled coir in growing media and wood and bark products imported from abroad or from UK plantations harbouring disease. There are also moderate risks from plant pathogens in Publicly Available Specification (PAS)100 organic waste composts in the absence of a requirement to compost to an industry standard. We discuss the implications of our findings for the future biosecurity of UK horticulture and make recommendations to limit plant pathogen risks, such as regulation for clarity on sources for all constituents, developing defined national standards for sanitisation regimes and routine testing for pathogens.

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